Image intensifier for optically produced images



LJHUQO 1.x LIN-HUI- Sept. 8, 1970 w. BAUMGARTNER 3,527,522

IMAGE INTENSIFIER FOR OPTICALLY PRODUCED IMAGES Filed March 11, 1968 4 Sheets-Sheet l Fig 1 PRIOR ART INVENTOR.

WI'LL Baurngcu'tner BY Ornegs Sept. 8, 1970 w. BAUMGARTNER 3,527,522

IMAGE INTENSIFIER FOR OPTICALLY PRODUCED IMAGES Filed March 11, 1968 4 Sheets-Sheet 3 INVENTOR. Br/l LL Bcwmgartner 0 megs Sept. 8, 1970 w. BAUM GARTNER 3,527,522

IMAGE INTENSIFIER FOR OPTICALLY PRODUCED IMAGES Filed March 11, 1968 4 Sheets-Sheet 5 Fig. 5

UT Fig. 7

- INVENTOR. \A/ILLJ Bavm arcner mi, csMmwwx, rmw

AtlZOF'HGUS p 3, 1970 w. BAUMGARTNER 3,527,522

IMAGE INTENSIFIER FOR OPTICALLY PRODUCED IMAGES Filed March ll. 1968 4 Sheets-Sheet 4 Fig. 6

INVENTOR.

\A/I LL Baumaart ner' I rheas United States Patent-O US. Cl. 350-161 9 Claims ABSTRACT OF THE DISCLOSURE An image intensifier for images that are optically produced in rastered form on a photo-conductive layer through an optical grid, the photo-conducting layer being adapted to control electrostatic forces used to deform a reflecting surface in accordance with the pattern of the rastered image. The reflecting surface is incorporated in a schlieren-optical light projection system having at least one diaphragm strip. The reflecting surface and the schlieren-optical system together serve to modulate a beam of a strong light source in accordance with the pattern of the rastered image in order to produce a similar intensified image. A first electrode grid which is associated with the photo-conductive layer, comprises an array of spaced parallel electrically conductive strips whose longitudinal edges extend perpendicular to the longitudinal edges of the diaphragm strip or strips and which are connected in alternating sequence to the poles of an electric voltage source. A second electrode grid comprises an array of spaced parallel electrically conductive strips Whose number, length, orientation and spacing of their longitudinal center-lines is the same as for the first electrode grid, and which likewise are connected in alternating sequence to the poles of a second electric voltage source. The second electrode grid is arranged spaced from the reflecting surface in such a manner that it compensates, by means of electrostatic fortfs at east a ima or a asic e ormation of the eflectin surface caused by the first e ectrode grid.

This invention relates to an image intensified for optically produced images.

Systems serving the same purpose are already known, and are described, for example, in United States Letters Pat. Nos. 2,896,507, 2,892,380 and 3,137,762. A further system of this type is described in co-pending United States patent application No. 494,540, filed Oct. 11, 1965.

As in the above-mentioned known systems, in the image intensifier of the present invention an optical image of at least one zone which is illuminated by a light source is produced on an associated strip-form diaphragm by reflection at a reflecting surface which is provided on a deformable layer and which is deformable together with said layer by electrostatic forces. The image to be intensified is imaged in raster form on a photo-conductive layer which influences an electrostatic field producing the deformation of the reflecting surface. Furthermore, an optical system is provided for observing the reflecting surface past the edges of the diaphragm strip or strips. The reflecting surface is preferably imaged on a projection screen on which an image corresponding to the image to be intensified is then visible with greater clarity.

US. Pat. No. 3,137,762 is concerned with a system of the above-mentioned type wherein an electrode grid is associated with said photo-conductive layer and comprises an array of spaced parallel electrically conductive strips whose longitudinal edges extend perpendicular to the longitudinal edges of the diaphragm strip or strips,

and which are connected in alternating sequence to the one and to the other pole of an electric voltage source.

Moreover, in this known arrangement, the photo-conductive layer is associated with an additional optical grid which includes opaque portions at least for certain light frequencies, the geometrical configuration of these opaque portions being different from that of the lines of the electrode grid.

It is an object of the present invention to provide an improved form of the last-mentioned system, and to reduce the demands that must be made on an optical image element used as one of the means for observing the reflecting surface.

For a better understanding of the significance of the present invention a description of a known image intensifier system will first be given with reference to FIGS. 1 to 4 of the accompanying drawings.

FIG. 1 is a schematic perspective representation of the complete system of a known image intensifier for optically produced images;

FIG. 2 illustrates the light-modulating unit of the system of FIG. 1, the unit being shown in greater detail and on a larger scale and with the component parts shown in a partially exploded manner;

FIG. 3 shows a bar zone forming part of the second schlieren diaphragm, and also the diffracted images of an aperture in the first schlieren diaphragm which occur during use of the system; and,

FIG. 4 is an illustration analogous to FIG. 3 which illustrates the disadvantage of the known system which is overcome by the image intensifier of the present invention.

As shown in FIG. 1, light emitted by an intense light source 1 is collimated by a condenser 2 and is thrown on to a first schlieren diaphragm 3. In order to simplify the following discussion the schlieren diaphragm 3 is assumed to be a simple perforated screen with a single circular aperture ,8 having a diameter of, for example, approximately 1 mm. A lens 4 receives light through the aperture and provides an image of the illuminated aperture B in the diaphragm 3 at infinity. After undergoing reflection in a light-modulating unit 5, which will be described in more detail later with reference to FIG. 2, the light rays are passed through a lens 6 which in combination with the action of the lens 4 provides an image B of the aperture 13 in the plane of a second schlieren diaphragm 7. This second schlieren diaphragm 7 includes a bar zone 7' which is opaque to light, and which is arranged such that the circular image [3 of the aperture {3 appears exactly on it. An objective lens 8 throws an image of a totally reflecting surface A, B, C, D (FIG. 2) contained within the light-modulating unit 5 on to a projection screen (not shown) which is positioned in the direc tion of the arrow P. A mirror 9 and an objective lens 10 pass a beam of light coming from an object in the direction of the arrow 0 on to a photo-conductive layer which is a component of the light-modulating unit 5. The objective 10 is so arranged that a sharp image of the object is optically produced on the photo-conductive layer, this object being for example a photographic transparency, a film, an illuminated object, etc.

The light-modulating unit 5 comprises an isosceles glass prism 11 having an apex angle of The light rays coming from the lens 4 fall perpendicularly on to one of the equal-length sides of the prism and pass through the prism 11 and through a glass backing plate 12. The latter is cemented to the base surface of the prism 11 in a reflection-free manner. A electrically conductive layer 13 which is transparent to light is carried on the other side of the glass plate 12 and may be, for example, a film or tin dioxide. On this layer 13 which acts as an electrode is mounted a layer 14 of a soft material, for example siliconised rubber. An air gap 15 of, for example, 50 micron width is left next to the layer 14 and then a block 16 is provided which is described in greater detail below with reference to FIG. 2. The light beam coming from the lens 4 traverses the glass plate 12 and also the layers 13 and 14, Whereafter it is totally refiected by the optically flat free surface of the layer 14 indicated by the reference letters A, B, C, D due to its angle of incidence of 45. After being reflected back through the layers 14 and 13, the glass plate 12 and the prism 11 the light beam passes through the lens 6.

In FIG. 2 the block 16 is shown split into its individual components. The different components 17 to 21 are shown in a partially exploded form for clarity, although in use they are fixedly secured in direct contact with one another. The block 16 includes a layer 17 of approximately 1 micron thickness which is formed of a photoconductive substance such as antimony sulphide. An electrode grid which consists of two interlaced comb-like grid elements 18a and 18b arranged with the grid lines of the two elements in alternating sequence is mounted on a carrier plate 19 which is formed from an electrically insulating and optically transparent material such as glass or quartz and which has a thickness of 100 to 200 microns. The grid elements 18a and 18b each comprise an array of parallel electrically conductive strips and are formed for example from aluminium deposited by evaporation in vacuo. The grid elements 18a and 18b are in electrical contact with the photo-conductive layer 17. Finally, the block 16 includes a line-form optical grid 20 which is composed of thin metal strips which are opaque to light and are equidistantly spaced, and which are carried on an optically transparent carrier plate 21. It is important that the lengthwise direction of the strips of the electrode grid elements 18a and 18b lies at rightangles to the longitudinal axis of the bar zone 7'. The lengthwise direction of the strips of the optical grid 20 lies at 45 to the direction of the strips of the electrode grid elements 18a and 18b. The two electrode grid elements 18a and 18b and also the optical grid 20 are shown on a greatly enlarged scale and in practice have a period spacing of, for example, 200 microns. The electrode grid elements 18a and 18b are connected to the two poles of an electric voltage source U such as a battery. A second voltage source U is connected between the electrically conductive layer 13 and electrode grid element 18a. The voltages of the two sources U and U may be, for example, 100 volts.

As is described in our British patent specification No.

987,867, the voltage source U generates an electric field in the air gap 15, and the strength of this field varies periodically in the direction parallel to the surface A, B, C, D and perpendicular to the longitudinal direction of the strips of the electrode grid elements 18a and 18b. This electric field, by generating electrostatic forces, causes a wave-like deformation of the surface A, B, C, D, hereinafter referred to as the basic deformation. The wave crests of the basic deformation lie parallel to the longitudinal direction of the strips of the electrode grid elements 18a and 18b. The additional use of the voltage source U amplifies the basic deformation without however affecting its periodicity and orientation.

The surface A, B, C, D which is deformed in this way represents a diffraction grating which acts as a mirror on account of the above-mentioned total reflection at the surface. Consequently, there appears in the plane of the schlieren diaphragm 7 besides the circular disc image provided solely by the plane surface A, B, C, D a number of further images {3 which are of the same size but different brightness. The centres of these images all lie on a straight line which runs perpendicular to the longitudinal direction-of the strips of the electrode grid elements 18a and 18b and which coincides with the longitudinal axis of the bar zone 7 (see FIG. 1). Since the bar zone 7' thus receives not only the original image 5' but also all the newly produced diffraction images 13", no light reaches the objective 8 and no light passes in the direction of the arrow P on to the projection screen. The basically deformed surface A, B, C, D is also invisible on the projection screen.

These conditions change however if a light beam is projected into the system in the direction of the arrow 0. As is described in United States patent specification No. 3,137,762 there then occurs in addition to the abovementioned basic deformation of the surface A, B, C, D a further deformation. The Wave crests of this additional deformation do not however coincide with the longitudinal direction of the strips of the electrode grid elements 18a and 18b. The superimposition of the two deformations results in an oblique angled woven type of pattern. This pattern appears at those portions of the surface A, B, C, D which are in alignment with illuminated portions of the photo-conductive layer 17. On the other hand, those portions of the surface A, B, C, D which are aligned with non-illuminated parts of the photo-conductive layer 17 are only subject to the basic deformation as before.

The surface A, B, C, D which is deformed in this twofold manner and which is equivalent to an oblique-angled reflecting cross-grating changes the distribution of the diffraction images in the plane of the schlieren diaphragm 7 in two different respects. On the one hand, new diffraction images 5 occur on both sides of the bar zone 7' (see FIG. 3 which illustrates the situation in the plane of the schlieren diaphragm 7), and, on the other hand, the brightness of the images B and a" already provided by the basic deformation is reduced since the light is partially transferred to the newly generated diffraction images 18'. Since the light forming the diffraction images 13" appearing on both sides of the bar zone 7 can pass to the objective 8 without being hindered by the bar, those parts of the surface A, B, C, D which possess the cross-grating pattern, i.e. which are aligned with the illuminated portions of the photo-conductive layer 17, are visible on the projection screen as bright areas.

The system does however pose a problem which the present invention overcomes. Even if the light beam projected along the line of the arrow 0 has a relatively small intensity, one nevertheless wishes to produce a bright image on the projection screen; this means that the voltages U and U must be sufficiently large. By calculation and experiment, it has been shown that voltages of such a magnitude cause a basic deformation of the layer 14 which is of such intensity that the most strongly diffracted of the images (3 and 5" are at a considerable distance (for example 5 cm.) from the undiffracted image ,3. This is illustrated in FIG. 4 in which in comparison to FIG. 3 the zone of greater brightness extends to diffraction images 9", 5" which lie far from the central image ,8. Some of the diffraction images f3", 6" even lie outside the aperture of the objective 8. If this light is also to be used for the image on the projection screen, then the objective 8 must also have a considerably greater diameter, with the results that the optical elements can become extremely expensive. The present invention on the other hand avoids such extreme displacement of the most strongly diffracted images by reducing the basic deformation, and can thus use an objective 8 of smaller aperture without adversely affecting the brightness of the image on the projection screen.

The system of the present invention has the first-mentioned constructional features of the known systems for intensifying an optically produced image. In known manner, the photo-conductive layer has adjacent to it an electrode grid which comprises an array of spaced parallel electrically conductive strips whose longitudinal edges extend at right-angles to the longitudinal edges of a stripform diaphragm and which are connected in alternating sequence to the one and to the other pole of an electric voltage source. The photo-conductive layer is furthermore mounted in front of an additional optical grid, which, at

least for certain light frequencies, has opaque portions whose geometrical configuration and arrangement is different from that of the strips of the electrode grid.

The novelty of the present invention essentially lies in the fact that a second electrode grid is provided comprising an array of spaced parallel electrically conductive strips whose number, length, orientation and spacing of their longitudinal centre-lines is the same as for the firstmentioned electrode grid, and which likewise are connected in alternating sequence to the one and the other pole of a second electric voltage source, and also in that the second electrode grid is arranged spaced from the reflecting surface in such manner that it compensates, by means of electrostatic forces, at least approximately for the basic deformation of the reflecting surface produced by said first electrode grid.

Other features and advantages of the present invention will become apparent from the following detailed description of two embodiments in accordance therewith which are given by way of example and with reference to FIGS. 5 to 7 of the accompanying drawings, in which:

FIG. 5 is a perspective view of the components of a light-modulating unit which replaces the corresponding known unit in the system of FIG. 1 and which is shown in more detail in FIG. 2, the unit being shown in partially exploded form for clarity;

FIG. 6 illustrates schematically the whole system of a second embodiment in accordance with the invention for intensifying an optically produced image; and,

FIG. 7 illustrates in greater detail the light-modulating unit of the system shown in FIG. 6.

The light-modulating unit illustrated in FIG. 5 and indicated generally by the reference 5a is constructed in exactly the same manner as the above-mentioned known unit shown in FIGS. 1 and 2 with respect to the elements 11, 12, 14, and 17 to 21, as well as the gap 15. However, it has the following differences. Instead of the electrically conductive electrode layer 13 of the known unit, a second electrode grid is provided which comprises two interlaced comb-like electrode grid elements 13a and 13b which are mounted on the transparent electrically insulating plate 12. The electrode grid comprises an array of spaced parallel electrically conductive strips which are alternately as sociated with one or the other comb-like grid elements 13a, 13b. The orientation, i.e. the longitudinal direction of the strips, of the electrode grid 13a, 13b is the same as that of the electrode grid 18a, 18b. Furthermore, the number and the length of the strips as well as the mutual spacing of the longitudinal centre-lines of the strips of both the electrode grids 13a, 13b and 18a, 18b correspond exactly.

The two electrode grid elements 13a and 13b may be produced for example by evaporation of aluminium in vacuo and are respectively connected to the one and to the other pole of an electric voltage source U for example a battery. The grid element 13b is also connected to one pole of the electric voltage source U whose other pole is connected to the grid element 18b.

Although in FIG. 5 for the sake of clarity the soft deformable layer 14 is shown slightly spaced from the plate 12 and from the second electrodegrid 13a, 13b, in practice it is located in direct contact with the electrode grid 13a, 13b. Likewise, the plate 21 carrying the optical grid 20 is in practice in direct contact with the plate 19 carrying the first electrode grid 18a, 18b. Only between the deformable layer 14 and the photo-conductive layer 17 is there a gap 15, as in the known arrangement of the light-modulating unit. Between the two electrode grids 13a, 13b and 18a, 1817, there is provided the deformable layer 14, the gap 15, and the photo-conductive layer 17. While the first electrode grid 18a, 18b is positioned in direct contact with the photo-conductive layer 17 and maintains an electric field therein, the second electrode grid 13a, 13b is positioned on the side of the deflecting surface A, B, C, D remote from the photo-conductive layer 17 The manner of operation of the light-modulating unit 5a of FIG. 5 which is used in the system of FIG. 1 in place of the known element 5 is as follows.

As long as the voltage sources U U and U are switched off, the free surface A, B, C, D of the deformable layer 14 facing the gap 15 is fiat. If only the two voltage sources U and U are switched on, then the surface A, B, C, D is deformed into a wave shape, just as was the case with the deformation of the known lightmodulating unit which was described with reference to FIGS. 1 and 2. If only the voltage source U is switched on, an electric field distribution defined by the second electrode grid 13a, 13b is generated in the interior of the layer 14 and likewise, due to electrostatic forces, leads to a wave-like deformation of the previously fiat surface A, B, C, D of the layer 14. In general, this deformation differs in amplitude from the above-mentioned basic deformation. On the other hand, the spacing of two adjacent wave peaks, i.e. the period of the deformation, is the same as for the basic deformation due to the same mutual spacing of the longitudinal centre-lines of the strips of both electrode grids 13a, 13b and 18a, 18b.

If all three voltage sources U U and U are switched on, a superimposition of the two deformations of the surface A, B, C, D takes place. The resulting total deformation is in general again wave-shaped, but its amplitude may be greated or smaller than the amplitude of each of the contributory deformations. If one displaces the plate 12 having the second electrode grid 13a, 13b mounted thereon in the plane of the plate in a direction perpendicular to the longitudinal direction of the strips of the grid 13a, 13b, then the resulting total deformation of the surface A, B, C, D will vary between a minimum and a maximum. Calculation and experiments show that a displacement of about half a period is necessary in order to change or reverse from maximum to minimum total deformation. The position of the plate 12 is thus chosen so that the total deformation of the surface A, B, C, D is a minimum. Then, by suitable selection of the ratios of the values of the two voltage sources U and U the magnitude of the total deformation can be further reduced. The remaining residual deformation consists of small wave-like curves in the surface A, B, C, D which have a period half as great as that of the contributory deformations and, most importantly, have only small amplitude.

Consequently, the basic deformation of the surface A, B, C, D produced by the first electrode grid 18a, 18b can be compensated at least approximately by the effect of the electrostatic forces generated by the second electrode grid 13a, 13b.

By means of this considerable compensation for the basic deformation of the reflecting surface A, B, C, D the diffraction images indicated in FIGS. 3 and 4 by 3" which arise with the layer 17 unilluminated remain in close proximity to the central image 5' of the diaphragm aperture 13. When light passes through the objective 10 and falls on to the photo-conductive layer 17 a new electric field distribution arises in the gap 15 which superimposes itself on the two fields which are produced by the electrode grids 18a, 18b and 13a, 13b and their associated voltage sources U and U A further deformation with waves dependent on the optical grid 20 is thus superimposed on the wave-shaped residual deformation of the surface A, B, C, D at those positions which correspond to illuminated parts of the photo-conductive layer 17. As a result, diffraction images 13" falling adjacent to the bar zone 7' are produced which cause an illumination of the projection screen. These diffraction images 3" which contribute to the image on the projection screen remain, unlike FIGS. 3 and 4, in the vicinity of the image 8, since the basic deformation of the surface A, B, C, D has been effectively compensated. Thus, the objective 8 (FIGS. 1, 3 and 4) does not need to have a particularly large aperture in order to encompass the diffraction images fi'. One can therefore avoid the necessity for an expensive objective of large diameter, without thereby adversely affecting the brightness of the image on the projection screen. The control of the image brightness in accordance with the original light distribution and the intensity of the image to be intensified which is incident on the layer 17 is not adversely affected by the abovementioned compensation for the basic deformation of the reflecting surface A, B, C, D.

The preceding description of the manner of operation of the light-modulating unit provided with a second electrode grid 13a, 1311 requires however some further amplification. If the light from the light source 1 has passed subsequently through the elements 2, 3, 4 and 12 of the system according to FIGS. 1 to 5, it strikes the second electrode grid 13a, 13b before it passes through the deformable layer 14 and is reflected at the surface A, B, C, D. After reflection the light again passes through the layer 14 and through the electrode grid 13a, 13b. The second passage of the light through the grid 13a, 13b is coupled with an additional diffraction effect which may possibly affect the functioning of the light-modulating unit. In order to keep this disturbance as small as possible, it is preferable to dimension the width of the individual strips of the electrode grid 13a, 13b to be small as possible, in consequence of which the optical transparency of the grid is as high as possible. For the same purpose, it is advantageous to make the electrode grid 13a, 1312 from a material of similar optical properties to the plate 12. Thus, for example, tin oxide is better than a metal.

It might be though that the diffraction effect caused by the second electrode grid 13a, 13b would perhaps contribute to a brightening of the image on the projection screen. This is not the case however since the strips of the two electrode grids 13a, 13b and 18a, 18b extend parallel to each other and at right-angles to the edges of the bar zone 7 of the second schlieren diaphragm 7, so that the diffraction images of the aperture 5 produced by the second electrode grid 13a, 13b likewise fall on the bar zone 7' just like the diffraction images 5 resulting from the basic deformation caused by the first electrode grid 18a, 18b. Consequently, the second electrode grid 13a, 13b produces no diffraction images of the aperture ,8 laterally of the bar zone 7 Thus, light only appears on the projection screen when a second electrode grid 13a, 13b is used when the photo-conductive layer 17 is illuminated by a light beam transmitted in the direction of the arrow 0, with the result that the brightness distribution on the projection screen corresponds to the light distribution on the photo-conductive layer 17.

The second embodiment in accordance with the present invention has the general arrangement illustrated in FIG. 6 which is already known except for the structure of the light-modulating unit 5b. An intense light source 1, for example an electric are light, has a condenser 2 associated therewith which focusses the light on to a mirror 22 which reflects the light on to a schlieren diaphragm 23. The latter consists of a plurality of parallel, opaque bars 23 in the form of strips which are spaced from one another and which fulfill two functions. The first function of the bars 23' is to form strip-like zones which are brightly illuminated by the light emanating from the light source 1 and which correspond functionally to the screen aperture B of the schlieren diaphragm 3 (FIG. 1) of the above-mentioned first embodiment. For this purpose, the sides of the bars 23 shown facing upwards in FIG. 6 are formed as reflecting surfaces. The second function of the bars 23' is to act as a strip-form zones which functionally correspond to the bar zone 7' of the schlieren diaphragm 7 (FIG. 1) of the first embodiment.

On the illuminated side of the bars 23 there is positioned an objective 4 and a light-modulating unit 5b whose construction will be described in greater detail below with reference to FIG. 7. The light-modulating By means of the objective 4 which is thus traversed twice by the light and by means of the above-mentioned reflecting surface of the unit 5b the illuminated bars 23 are imaged on the bars 23' which now act as diaphragm strips.

A projection objective 8 and a mirror 24 are positioned on the unilluminated side of the bars 23 in such manner that they produce, through the gaps between the bars 23', images of the reflecting surface of the light-modulating unit 5b, which are projected in the direction of the arrow P on to a screen (not shown). In this imaging process the bars 23' solely act as strip diaphragms which are not reproduced on the projection screen.

A further mirror 9 and an objective 10 act to produce optically an image from light rays transmitted in the direction of the arrow -0 on a photo-conductive layer of the light-modulating unit 5b, said image being the image to be intensified by the system and coming, for example, from a transparency, a film, or an actual object, etc.

The light-modulating unit 5b has the construction shown in FIG. 7 and differs in some respects from the known units. Like the light-modulating unit of FIG. 5 this unit 5b comprises a carrier plate 12, an electrode grid 13a, 1312, a photo-conductive layer 17, an electrode grid 18a, 1812, a carrier plate 19, an optical grid 20, and a carrier plate 21. Also, the arrangement of the known elements with respect to each other is the same as in the light-modulating unit 5a of FIG. 5. On the other hand, however, the light-modulating unit 5b of FIG. 7 lacks the prism 11 of FIG. 5 and is provided with two electrically insulating, elastically deformable layers 24 and 25 and an intermediate mirror layer 26, these three layers being positioned between the electrode grid 13a, 13b and the photoconductive layer 17. The thickness of each deformable layer 24 and 25 is approximately microns, and these layers themselves are preferably formed from a silicon polymer with an added plasticiser, for example siliconised rubber with plasticiser, and preferably have a modulus of elasticity between 10 and 10 dynes/cmF. The mirror layer 26 has a thickness of 10 to 60 microns and consists of a material which is liquid in the operating state of the unit, for example a metal or a metal alloy. If the unit is to be used at normal room temperatures then the mirror layer 26 is preferably formed from mercury or an amalgam with a small amount, approximately 1% by weight, of indium. All the above-mentioned plates and layers of the light-modulating unit 5b are arranged with no intermediate spaces or gaps between them, although in FIG. 7 some of the elements are illustrated as being spaced from one another for the sake of clarity.

The two grid elements 18a and 18b of the electrode grid 18a, 18b are respectively connected to one and the other pole of a DC. voltage source U In an analogous manner, the two grid elements 13a and 13b of the second electrode grid 13a, 13b are connected to a DC voltage source U An AC. voltage source U has its one terminal connected to the electrically conductive mirror layer 26 and its other terminal connected to the electrode grid element 18b. A second AC. voltage source U is connected between the mirror layer 26 and the electrode grid element 13b. The AC. voltages from the sources U and U have the same frequency and phase, but their amplitudes may be different.

The manner of operation of the system as shown in FIGS. 6 and 7 will now be described.

For a better understanding of the system it will first be assumed that the voltage sources U and U;,' are not connected in circuit and that the voltage of the source U is zero. If no light falls on the photo-conductive layer 17 in the direction corresponding to the arrow 0 then the electrostatic field between the electrode grid 18a, 18b and the mirror layer 26 acting likewise as an electrode is essentially homogeneous, if one disregards the edge effects and the strip form of the grid 18a, 18b. The electrostatic forces acting on the surface I between the mirror layer 26 and the deformable layer 25 are consequently of the same magnitude at all positions on the surface I, with the result that the latter undergoes no deformation and remains flat. The conditions are unaltered by the fact that the voltage source U provides an A.C. voltage. This causes only a time-wise not a spatial variation in the electrostatic forces at the surface I. If the DC. voltage of the source U is not zero, a spatially varying field is superimposed on the above-mentioned time-varying electric field, with the result that the distribution of the field forces at the surface I is unhomogeneous and the surface I of the mirror layer 26 undergoes a wave-shaped basic deformation, with the wave crests and wave troughs extending parallel to the strips of the electrode grid 18a, 18b and perpendicular to the bars 23' of the schlieren diaphragm 23.

During the time taken for the build-up of this basic deformation, i.e. before the field forces acting at the surface I are counterbalanced by the elastic reaction forces of the layer 25, pressure waves and displacement waves are propagated from the surface I transversely through the liquid mirror layer 26. On the arrival of these pressure waves and displacement waves at the opposite surface II a pressure distribution is set up which is point for point analogous with the electrostatic field distribution at the surface I. Consequently, the surface II is deformed in a wave-shaped manner in the same way as the surface I. The comparatively large internal friction of the deformable layers 24 and 25 ensures a suflicient damping of any mechanical reasonance of the layers and of the liquid mirror layer 26. The surface II of the mirror layer 26 which is deformed in this way acts as a diffraction grating for the light rays coming from the light source 1 and reflected at the surface II. The resulting diffraction images of the illuminated bars 23' are displaced in the longitudinal direction of the bars and thus cause no visible brightening on the projection screen.

If an image to be intensified is produced by light rays passing in the direction of the arrow on to the photoconductive layer 17, the illuminated parts of the layer 17 cause a spatial change in the electrostatic field distribution between the electrode grid 18a, 18b and the mirror layer 26 so that the above-mentioned basic deformatior of the two surfaces I, II of the mirror layer has a furthe deformation superimposed thereon. This results in diffraction images of the illuminated bars 23' which fall laterally adjacent to the bars and cause an illumination of the projection screen at those positions which correspond to the illuminated portions of the photo-conductive layer 17. On account of the above-mentioned basic deformation of the mirror layer 26 the centres of the diffraction images which are thus produced are at a comparatively large distance from the centres of the bars 23', with the result that the objective 8 must have a large diameter in order to completely catch the light of the diffraction images and transmit it to the projection screen. This disadvantage can be avoided by use of the second electrode grid 13a, 13b and the voltage sources U and U The effect of the electrode grid 13a, 13b and the voltage sources U and U on the mirror layer 26 basically corresponds to the above-mentioned action of the electrode grid 18a, 18b and the voltage sources U and U with the photo-conductive layer 17 unilluminated. The position of the second electrode grid 13a, 13b is chosen and the ratio of the amplitudes of the A.C. voltages from the sources U and U,;, is set such that by means of the electrostatic field effect caused by the second electrode grid 13a, 13b the basic deformation of the mirror layer 26 produced by the other electrode grid 13a, 13b is compensated as far as possible. Apart from the remaining residual deformation which cannot be compensated, the mirror layer 26 is then essentially only deformed by the changes in field which are produced by illumination of the photo-conductive layer 17. The centres of the resulting diffraction images of the illuminated surfaces of the bars which fall adjacent to the bars 23 then lie in closer proximity to the centres of the bars than in the case where a basic deformation of the mirror layer 26 exists. The aperture of the objective 8 need no longer be so large in order to catch the light from the diffraction images and transmit it to the projection screen,.with the result that an objective which is less expensive can be used.

It should perhaps be pointed out that the two voltage sources U and U are A.C. voltage sources since the mirror layer 26 is formed by a liquid which must reproduce point for point on the surface II only time-wise changing deformations of surface I in controlled dependence on the photo-conductive layer 17. The mirror layer 26 must also be continuously maintained in pulsating movement. The brightness intensity of the illuminated areas on the projection screen changes periodically between zero and a maximum value in rhythm with the surface II. If this rhythm is sufficiently fast and is for example of the order of 20 cycles per second then no flickering of the images on the projection screen is visible by an observer on account of the speed of response of the retina of the eye. Further forms of light-modulating unit with a liquid mirror layer are disclosed in the abovementioned United States patent application No. 494,540.

I claim:

1. An image intensifier for optically produced images wherein an optical image of at least one zone illuminated by a light source is produced on an associated strip-form diaphragm y QggggflksfiWM provided on a deforma e ayer and which is deformable :W wherein a p o o-con uc we a r 18 provided on which the image to be intensified is imaged in raster form and which influences an electrostatic field producing the deformation of said reflecting surface, wherein an optical system is provided for observing said reflecting surface past the edges of the diaphragm strip or strips, wherein a first electrode grid is provided in association with said photoconductive layer, said electrode grid comprising an array of spaced parallel electrically conductive strips whose longitudinal edges extend perpendicular to the longitudinal edges of the diaphragm strip or strips and which are connected in alternating sequence to the one and to the other pole of an electric voltage source, and wherein an optical grid is provided between said photo-conductive layer and the image source and includes at least for certain light frequencies opaque portions whose geometrical configuration is dilferent from that of the lines of said electrode grid, characterized in that a second electrode grid is provided comprising an array of spaced parallel electrically conductive strips whose number, length, orientation and spacing of their longitudinal center-lines is the same as for said first electrode grid, and which likewise are connected in alternating sequence to the one and to the other pole of a second electric voltage source, and also in that said second electrode grid is arranged spaced from said reflecting surface in such manner that it compensates, by means of electrostatic forces, at least approximately for the basic deformation of the reflecting surface produced by said first electrode grid.

2. An image intensifier as claimed in claim 1, wherein said second electrode grid is positioned on the side of the reflecting surface remote from said first electrode grid.

3. An image intensifier as claimed in claim 2, wherein the photo-conductive layer, the deformable layer with the reflecting surface thereon, and an intermediate gap formed between the photo-conductive layer and the reflecting surface are all positioned between said first and second electrode grids.

4. An image intensifier as claimed in claim 3, wherein the first electrode grid and the photo-conductive layer are arranged in electrical contact with each other on a first transparent plate of electrically insulating material, and wherein the second electrode grid and the deformable layer are mounted on a second transparent plate of electrically insulating material, the two electrode grids being respectively positioned on the facing surfaces of the two transparent plates which are spaced from each other.

5. An image intensifier as claimed in claim 1, wherein a third electric voltage source is connected on the one hand to one pole of the first voltage source and On the other hand to one pole of the second source. I

6. An image intensifier as claimed in claim 1, wherein said reflecting surface is formed by a layer of electrically conductive material which is deformable together with said deformable layer, wherein a third electric voltage source is connected on the one hand to one pole of the first voltage source and on the other hand to the reflecting layer, and wherein a fourth electric voltage source is connected on the one hand to one pole of the second voltage source and one the other hand to the reflecting layer.

7. An image intensifier as claimed in claim 6, wherein the reflecting layer is formed by a material which is liquid in the operating state of the system and which is located between two electrically insulating elastically deformable layers, and wherein said third and fourth voltage sources are AC. voltage sources having the same frequency and phase.

8. An image intensifier as claimed in claim 7, wherein the first electrode grid and the photo-conductive layer are arranged in electrical contact with each other on a first transparent plate of electrically insulating material, wherein the second electrode grid is mounted on a second transparent plate of electrically insulating material, said two electrode grids being respectively positioned on the facing surfaces of the two transparent plates which are spaced from each other, and wherein the reflecting layer and said two electrically deformable layers are positioned between the photo-conductive layer and said second electrode grid.

9. An image intensifier as claimed in claim 1, wherein said second electrode grid is at least partially permeable to light.

References Cited UNITED STATES PATENTS 3,137,762 6/ 1964 Baurngartner et a1.

RONALD L. WIBERT, Primary Examiner W. L. SIKES, Assistant Examiner 

